The control of animal behavior depends on the precise interactions of a large number of neurons. The mechanisms are not known which give rise to the specific connections between neurons to form functional neural networks. Most explanations demand that there be quantitative or qualtitative chemical differences between neurons which either lead to or result from the formation of specific connections. Monoclonal antibodies directed against antigens in the simple nerve cord of the leech in many cases only bind to a single or a few neurons of the four hundred cells in each leech ganglion. The high frequency of these specific antibodies and the lack of overlap between different antibodies suggests the nervous system may be an extremely heterogeneous tissue. If this heterogeneity is functionally significant then hybridoma technology offers a powerful method of studying the assembly and functioning of integrated neural networks. The general ability which monoclonal antibodies provide of defining chemicals restricted to a few neurons is already being widely applied in neurobiology. Two strategies have been used. Antibodies have been raised to particular molecules of known importance of alternatively antibodies have been raised against a complex immunogen and specific antibodies sought. This proposal falls into this second class using the leech where the anatomy, electrophysiology and development are relatively well understood. This simplifies the search for specific antibodies. The aim is to use the antibodies to define neural networks in the ultimate hope of understanding the molecular events which differentiate neurons into these functionally discrete groups.